Much of your post is about the controversies arising from U.S. enforcement of the embargo to restrict actions taken by foreign subsidiaries of U.S. corporations. Is that overreaching? I don't think it would be seen that way by most DUers if the underlying policy were one we supported. In that case, we'd be clear that U.S. corporations shouldn't be able to evade the law by acting through a foreign subsidiary.
For example, Walmart has been slammed because it took no significant action upon learning that Wal-Mart de Mexico had been routinely paying bribes to government officers to obtain construction permits. That's a violation of the Foreign Corrupt Practices Act. (See the New York Times story "Vast Mexico Bribery Case Hushed Up by Wal-Mart After Top-Level Struggle" for the details.)
You quote a news report about a Cuban-embargo case against a Canadian citizen: "The Canadian government has criticised the United States over the charges filed against Mr Sabzali, saying it was trying to impose US law outside its own borders." I don't think that's a valid objection. It's reasonable for the U.S. government to try to regulate the actions of multinational corporations that are based here, such as Walmart. The valid objection is that the underlying policy, whether applied to foreign subsidiaries or to actions taken within the U.S., is a bad one in the case of the Cuban embargo (even though it's a good one in the case of the FCPA).
As to your final point, I know that the embargo is widely called a "blockade" by its opponents in Latin America and elsewhere. Those of us who believe that words have meanings will continue to disagree with them. (A story attributed to Abraham Lincoln: "If you call a tail a leg, how many legs does a sheep have? The answer is four. Calling a tail a leg doesn't make it a leg."
One wonders what they would say if the U.S. Navy were to start intercepting ships. I would say "U.S. institutes blockade" but people who've said there was already a blockade couldn't consistently describe the action that way. They might find that they had cheapened the word "blockade" by using it where it didn't apply.